The present invention relates to drag-reducing polymers and their method of manufacture. More specifically, this invention relates to a method for preparing ultra-high molecular weight, hydrocarbon-soluble polymers capable of dissolving even in cold fluids.
A drag-reducing agent is one that substantially reduces the friction loss that results from the turbulent flow of a fluid. Where fluids are transported over long distances, such as in oil and other hydrocarbon liquid pipelines, these friction losses result in inefficiencies that increase equipment and operations costs. Ultra-high molecular weight polymers are known to function well as drag-reducing agents, particularly in hydrocarbon liquids. In general, drag reduction depends in part upon the molecular weight of the polymer additive and its ability to dissolve in the hydrocarbon under turbulent flow. Effective drag-reducing polymers typically have molecular weights in excess of five million.
Drag-reducing polymers are known in the art. Representative, but non-exhaustive, samples of such art are: U.S. Pat. No. 3,692,676, which teaches a method for reducing friction loss or drag for pumpable fluids through pipelines by adding a minor amount of a high molecular weight, non-crystalline polymer; and U.S. Pat. No. 3,884,252, which teaches the use of polymer crumb as a drag-reducing material. These materials are extremely viscoelastic, and, in general, have no known use other than as drag-reducing materials. However, the very properties that make these materials effective as drag-reducing additives make them difficult to handle because they have a severe tendency to cold flow and reagglomerate even at subambient temperatures. Under conditions of pressure, such as stacking or palleting, cold flow is even more intense and reagglomeration occurs very quickly.
The general propensity of non-crosslinked elastomeric polymers (elastomers) to cold flow and agglomerate is well-known. Polymers of this sort cannot be pelletized or put into discrete form and then stored for any reasonable period of time without the materials flowing together to form large agglomerates. Because of such difficulties, elastomers are normally shipped and used as bales of rubber. However, such bales must be handled on expensive equipment and cannot be pre-blended. In addition, polymers such as the drag-reducing additives described are not susceptible to such balings, since cold flow is extremely severe. Further, dissolution time for such drag-reducing materials from the bulk polymer state in the flowing hydrocarbons to a dissolved state is so lengthy as to severely reduce the effectiveness of this material as a drag-reducing substance.
Numerous attempts have been made to overcome the disadvantages inherent in cold-flowing polymers. Representative, but non-exhaustive, of such art is that described in U.S. Pat. No. 3,791,913, wherein elastomeric pellets are surface cured, i.e., vulcanized to a minor depth in order to maintain the unvulcanized interior of the polymer in a xe2x80x9csackxe2x80x9d of cured material, and U.S. Pat. No. 4,147,677, describing a method of preparing a free-flowing, finely divided powder of neutralized sulfonated elastomer by admixing with fillers and oils. This reference does not teach a method for making free-flowing powders of non-elastomeric material. U.S. Pat. No. 3,736,288 teaches solutions of drag-reducing polymers in normally liquid vehicles for addition to liquids flowing in conduits. A xe2x80x9cstaggered dissolutionxe2x80x9d effect is provided by varying the size of the polymer particles. Suspension or surface-active agents can also be used. While directed to ethylene oxide polymers, the method is useful for hydrocarbon-soluble polymers as well. U.S. Pat. No. 4,088,622 describes a method of making an improved, molded drag-reducing coating by incorporating antioxidants, lubricants, and plasticizers and wetting agents in the form of a coating which is bonded directly onto the surface of materials passing through a liquid medium. U.S. Pat. No. 4,340,076 teaches a process for dissolving ultra-high molecular weight hydrocarbon polymer and liquid hydrocarbons by chilling to cryogenic temperatures comminuting the polymer formed into discrete particles and contacting these materials at near cryogenic temperatures with the liquid hydrocarbons to more rapidly dissolve the polymer. U.S. Pat. No. 4,341,078 immobilizes toxic liquids within a container by injecting a slurry of cryogenically ground polymer particles while still at cryogenic temperatures into the toxic liquid. U.S. Pat. No. 4,420,440 teaches a method for collecting spilled hydrocarbons by dissolving sufficient polymer to form a nonflowing material of semisolid consistency by contacting said hydrocarbons with a slurry of cryogenically comminuted ground polymer particles while still at cryogenic temperatures.
Some current drag-reduction systems inject a drag-reducing polymer solution containing a high percentage of dissolved, ultra-high molecular weight polymer into conduits containing the hydrocarbon. Another way to introduce ultra-high molecular weight polymers into the flowing hydrocarbon stream is through a suspension. Powdered ultra-high molecular weight polymers are suspended in a liquid that will not dissolve or will only partially dissolve the ultra-high molecular weight polymer. This suspension is then introduced into the flowing hydrocarbon stream.
Polyalphaolefin drag-reducing polymers, when produced in bulk polymerization processes or when reduced to high solids content, such as when precipitated from solution, can exhibit regions within the polymer which have a high level of order. These regions of high order, including phenomenon such as crystallinity, are often a very small fraction of the overall polymer. However, these regions may have a dramatic effect on the ability to dissolve the drag-reducing polymer. The regions of high order act somewhat like knots in a ball of yarn and are much slower to solvate and dissolve when the drag-reducing polymer is mixed into the hydrocarbon. These regions of high order may be increased by cryogenic processing, such as in cyrogenic grinding. The regions of high order may also increase during cold storage of the polymers.
What is needed is a drag-reducing polymer which exhibits very little or no regions of high order in the solid state. Further, these regions should not return during cold storage.
Accordingly, a drag-reducing polymer and drag-reducing polymer suspension and method of manufacturing the drag-reducing polymer suspension are disclosed herein. One embodiment of the present invention is drawn to an ultra-high molecular weight copolymer comprising an alpha-olefin monomer with carbon chain lengths of between four and nine carbons and having less than 25% monomers with carbon chain lengths of twelve carbons or longer. The ultra-high molecular weight copolymer has a molecular weight of greater than 1 million and should have substantially no reluctant fraction as measured by the Cold-Shear Warm Filtration Test.
In another embodiment, a method of manufacturing an ultra-high molecular weight copolymer is disclosed. A mixture of an ultra-high molecular weight copolymer comprising an alpha-olefin monomer with a carbon chain length of between four and nine carbon atoms and having less than 25% monomers with carbon chain lengths of twelve or longer is bulk polymerized to form the ultra-high molecular weight copolymer. The ultra-high molecular weight copolymer has a molecular weight of greater than 1 million. In still another embodiment, a method of preparing a drag-reducing copolymer suspension is disclosed where the ultra-high molecular weight copolymer is manufactured by bulk polymerizing a monomer mixture composed of a majority of alpha-olefin monomers with carbon chain lengths of between 4 and 9 carbons and less than 25% monomers with carbon chain lengths of twelve or longer, wherein the ultra-high molecular weight copolymer has a molecular weight of greater than 1 million and substantially no reluctant fraction, and then mixing the ultra-high molecular weight copolymer with a suspending fluid. An initiator, catalyst and promoter are then added and the mixture allowed to react to form the ultra-high molecular weight copolymer. The ultra-high molecular weight copolymer has a molecular weight of greater than 1 million.
In still another embodiment of the present invention, a method for the preparation of a drag-reducing copolymer suspension is disclosed where an ultra-high molecular weight copolymer is prepared by solution copolymerization of a monomer mixture composed of a majority of alpha-olefin monomers with carbon chain lengths of between 4 and 9 carbons and less than 25% monomers with carbon chain lengths of twelve or longer, wherein the ultra-high molecular weight copolymer has a molecular weight of greater than 1 million and substantially no reluctant fraction, separating the ultra-high molecular weight copolymer from the hydrocarbon solvent and then mixing the ultra-high molecular weight copolymer with a suspending fluid. The ultra-high molecular weight copolymer may be ground at a temperature below the glass-transition temperature of the ultra-high molecular weight copolymer to form ground polymer particles prior to mixing the copolymer with the suspending fluid.
One advantage of the present invention is that the drag-reducing polymer suspension is easily transportable and does not require pressurized or special equipment for storage, transport, or injection. Another advantage is that the drag-reducing polymer is quickly dissolved in various flowing hydrocarbon streams. Yet another advantage is that the copolymers of the present invention are fully soluble in heavy aromatic hydrocarbons at temperatures as low as 0xc2x0 C. (32xc2x0 F.). Another advantage of copolymers of the present invention is that they may either be transported in a stable suspension, or pre-dissolved in a small amount of heavy hydrocarbon. Still another advantage of the present invention is that reagglomeration of the drag-reducing polymers is greatly reduced, allowing for easier handling during manufacture. Another advantage of the present invention is that the drag-reducing polymer suspension is stable, allowing a longer shelf life and balancing of customer demand with manufacturing time.